This work starts from Eriksson's theoretical conclusions leading to an isotherm based on a, law of mass action type, relation. The activity coefficients at the interface are expressed as for regular bulk solutions. This leads to an isotherm of the type given by Bockris and Swinkels and to a definition of the interaction coefficient. The basic hypotheses and simplifications used in the evaluation of the degree of coverage are discussed, as well as the various approximations generally envisaged. A critical evaluation is made of the comparison between the apparent free energies of adsorption of a homologous series of aliphatic compounds and the conclusions drawn from these results. © 1979 Elsevier Sequoia S.A.